The present invention relates to the improvement of manufacturing process of semi-insulating gallium arsenide single crystal to be applied as substrates of optical elements or magnetic elements.
Semi-insulating gallium arsenide single crystals are applied as substrates of various microwave elements, optical elements, magnetic elements, etc. In these years, they have been put into practical use for integrated circuits (IC) and large scale integrated circuits (LSI) since the techniques have been developed for ion implantation into such substrates. A certain degree of thermal treatment is required in the process of ion implantation, and therefore the substrates are required to have thermally stable electric properties. Moreover, in case that they are applied as substrates for IC or LSI, they should have much higher resistivity than that of conventional semi-insulating materials; i.e. 10.sup.7 -10.sup.8 .OMEGA..cm and over at 300.degree. K.
In order to satisfy the above requirements, the following processes have been adopted conventionally:
(1) Process for growing chrome doped single crystal PA0 (2) Process for growing oxygen doped single crystal PA0 (3) Process for growing undoped single crystal having low impurity concentration
Among the above three processes, the (1) and (2) processes are the ones to compensate electrically the conductive levels by doping, which are of impurities of Si and Cu, of vacancies of Ga and As and of compounds of impurities and vacancies.
It is difficult to control doping quantities of chrome by the above (1) process, because the segregation coefficient of chrome to gallium arsenide is so small as about 6.times.10.sup.-4 and the doping of chrome into gallium arsenide crystal becomes difficult. Moreover, it has such problems that, if excessive chrome is doped for the purpose to stabilize the electric properties of the crystal, dislocation and precipitates are observed in the crystal and that, if small quantity of chrome is doped, the crystal becomes thermally instable.
It is hard to carry out doping control in the above (2) process and the crystal has low thermal stability. As a technique to practice the above (3) process, a direct synthesized liquid encapsulated pulling method is described in the following reference: Pekarek et. al : Czech. J. Phys., 20 (1970). The details of the above method will be described later. The crystal obtained by the conventional liquid encapsulated pulling method contains much impurities, because this method adapts the separately synthesized gallium arsenide as its starting material. In contrast, the crystal obtained by the direct synthesized liquid encapsulated pulling method contains less impurities and is so-called "intrinsic semiconductor", because this method synthesizes gallium arsenide by reacting directly gallium and arsenic and from its melt grows crystal under high pressure. Under the present conditions, however, crystals developed by this method have high impurities content and the necessity of high pressure makes an inner construction of a furnace different from the conventional one and makes crystal growth process difficult. Moreover, there are many technical problems to be solved such as high density of dislocation and shift from stoichiometry, which is common with the conventional pulling method.